The term 'Jack of a Nursery Rhyme' is a reference to the character Jack from the nursery rhyme 'Jack and Jill'. Jack is the protagonist of the rhyme, who goes up a hill to fetch a pail of water. The crossword solution for this term is 'SPRAT', which is a reference to the line in the nursery rhyme 'Jack SPRAT could eat no fat'. This line is used to describe Jack's eating habits, as he was able to eat lean food while his wife, Jill, was able to eat fatty food.